By using a principle, that is, a piezoelectric effect, of a piezoelectric ceramic (hereinafter, also referred to as “piezoelectric ceramic composition” in some cases) in which electrical energy is converted into mechanical energy or mechanical energy is converted into electrical energy, piezoelectric ceramics have been used for many electronic devices.
Heretofore, in a piezoelectric device including a piezoelectric ceramic, for example, there are used a lead-containing piezoelectric ceramic composition composed of two components represented by PbTiO3—PbZrO3 (hereafter referred to as “PZT”) and PZT containing as a third component a lead-containing composite perovskite composition represented by Pb(Mg1/3Nb2/3)O3, Pb(Zn1/3Nb2/3)O3, or the like.
These lead-containing piezoelectric-ceramic compositions containing PZT as a primary component have high piezoelectric properties and are used for most electronic devices using the piezoelectric effect which are now in practical use.
However, since lead is contained in the primary component, high environmental burdens, such as volatilization of PbO in manufacturing, have caused problems.
Accordingly, a piezoelectric ceramic composition containing no lead or a small amount of lead has been desired.
In recent years, research and development has been energetically carried out, and as piezoelectric ceramic compositions containing no lead, for example, there may be mentioned a composition having a perovskite structure composed of BaTiO3 (Non-Patent Documents 1 and 2), a composition having a Bi-containing perovskite structure composed of two components represented by (Bi1/2Na1/2)TiO3—(Bi1/2K1/2)TiO3 (Patent Documents 1 to 4), a composition having a tungsten bronze structure which contains (Ba, Sr, Ca)2NaNb5O15 as a primary component (Patent Documents 5 to 7), a composition having a bismuth layer structure which contains SrBi2Nb209 as a primary component (Patent Documents 8 to 10), and a composition having an alkali-containing niobic acid-perovskite structure which contains KNbO2—NaNbO2—LiNbO2 (Patent Documents 11 to 13) as a primary component.    Non-Patent Document 1: Japanese Journal of Applied Physics Vol. 45, No. 1, 2006, pp. L30 to L32    Non-Patent Document 2: Japanese Journal of Applied Physics Vol. 46, No. 4, 2007, pp. L97 to 98    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-201172    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-75449    Patent Document 3: Japanese Patent No. 4044943    Patent Document 4: Japanese Patent No. 4044944    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-75448    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2004-161532    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2006-143588    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2001-130961    Patent Document 9: Japanese Unexamined Patent Application Publication No. 2002-241175    Patent Document 10: Japanese Unexamined Patent Application Publication No. 2006-62955    Patent Document 11: Japanese Unexamined Patent Application Publication No. 2006-56778    Patent Document 12: Japanese Unexamined Patent Application Publication No. 2007-204336    Patent Document 13: Japanese Unexamined Patent Application Publication No. 2004-244301
Among the above compositions, the compositions described in Non-Patent Documents 1 and 2 are each a piezoelectric ceramic composition that uses BaTiO3, which has been used in large amounts for ceramic capacitors and the like and to which it is believed that existing techniques are easily applied, which exhibits high piezoelectric properties, and which has a homogeneous polycrystalline structure. However, since the Curie temperature of BaTiO3 is approximately 125° C., as the limit by this physical property, the piezoelectric effect disappears in a temperature range of 125° C. or more, and hence the above composition is not appropriately applied to a piezoelectric device used, for example, in a vehicle, which is required to work in a wide temperature range of −50° C. to 150° C.
In addition, in the compositions described in Patent Documents 1 to 4, a temperature at which the composition becomes an antiferroelectric crystal and loses its piezoelectric effect is in a range of 100° C. to 200° C., and hence, these compositions are also not appropriately used for operation in a high temperature range of 100° C. or more.
In addition, since having a Curie temperature of approximately 300° C., the compositions described in Patent Documents 5 to 7 each exhibit the piezoelectric effect in a wide temperature range of −50° C. to 150° C.
However, in the composition described in Patent Document 5, the electromechanical coupling coefficient (kr), which is the index of the piezoelectric properties, of a disc element in the diameter direction is as low as 12.5% at the maximum.
Furthermore, in the compositions described in Patent Documents 6 and 7, the polycrystalline structures thereof each have an average diameter of 3 to 20 μm, and hence, for application to recent piezoelectric devices which are required to have a smaller size, a smaller layer thickness, and smaller film thickness, the compositions described above are required to have a more homogeneous polycrystalline structure.
In addition, because they have low piezoelectric properties, the compositions described in Patent Documents 8 to 10 are also not appropriately used as a substitute for PZT.
In addition, since the compositions described in Patent Documents 11 to 13 which are similar to the composition of the present invention are able to have a high Curie temperature of 250° C. or more and to have a piezoelectric displacement characteristic (−d31) of 100 or more in a horizontal direction along a polarization direction when the composition is appropriately selected, it is well known that the compositions described above have excellent properties as a piezoelectric ceramic composition containing no lead.
However, as described in Patent Document 12, the polycrystalline structure of the piezoelectric composition contains large grains having a grain diameter of up to 20 μm, and hence, for application to recent piezoelectric devices which are required to have a smaller size, a smaller layer thickness, and a smaller film thickness, the use of the composition described above is inherently limited to a certain degree as described above.
In addition, although the compositions described in Patent Documents 11 and 13 each attempted to improve its sintering properties and piezoelectric properties by addition of SiO2 to its main composition, a K3Nb3O6Si2O7 crystalline phase has not been disclosed. Unlike the composition of the present invention [in paragraph [0136] of Patent Document 13, it has only been suggested that SiO2 used as an additive is partly contained in grain boundaries of each piezoelectric ceramic composition as an oxide or a compound, such as a perovskite structure compound and is partly contained so that a Si atom substitutes for at least part of Nb, Ta, and Sb of a compound represented by {Li0.04(K0.5Na0.5)0.96} (Nb0.86Ta0.1Sb0.04)O3]. Furthermore, the polycrystalline structure of the composition has also not been disclosed; hence, the present invention is a completely novel invention.
As for a piezoelectric composition represented by a general formula [Na1-xKx]1-yLiy[Nb1-z-wTazSbw]O3 which is well known as a piezoelectric ceramic composition having an alkali-containing niobic acid-perovskite structure and containing no lead, by a conventional method, a polycrystalline structure which has a crystal having a maximum grain diameter of 10 μm or more is inevitably formed.
On the other hand, in order to suppress the precipitation of a crystal having a size of 10 μm or more, improvements by specific methods, such as a decrease in firing temperature by HIP (Non-Patent Document 3), firing in a K20 atmosphere (Patent Document 14), significantly precise control of a firing speed and a firing temperature (Non-Patent Document 4) have been attempted; however, there has been a limit.    Non-Patent Document 3: Journal of American Ceramics Society Vol. 45, No. 5, 209-213 (1962)    Non-Patent Document 4: Materials Science and Engineering B 131 83-87 (2006)    Patent Document 14: Japanese Unexamined Patent Application Publication No. 2004-359539